Liquid crystal displays (LCDs) have been used in not only TV sets with a big screen but also small display devices such as the monitor screen of a cellphone. In an LCD, one pixel consists of three subpixels representing red (R), green (G) and blue (B) that are the three primary colors of light, and the difference in color between those red, green and blue subpixels is typically produced by color filters.
TN (twisted nematic) mode LCDs, which would often be used in the past, achieved relatively narrow viewing angles, but LCDs of various other modes with wider viewing angles have recently been developed one after another. Examples of those wider viewing angle modes include IPS (in-plane switching) mode and VA (vertical alignment) mode. Among those wide viewing angle modes, the VA mode is adopted in a lot of LCDs because the VA mode would achieve a sufficiently high contrast ratio.
When viewed obliquely, however, the VA mode LCD sometimes produces grayscale inversion. Thus, to minimize such grayscale inversion, an MVA (multi-domain vertical alignment) mode in which multiple liquid crystal domains are defined within a single pixel region is adopted. In an MVA mode LCD, an alignment control structure is provided for at least one of the two substrates, which face each other with a vertical alignment liquid crystal layer interposed between them, so that the alignment control structure contacts with the liquid crystal layer. As the alignment control structure, a linear slit (opening) of an electrode or a rib (projection) may be used, thereby applying anchoring force to the liquid crystal layer from one or both sides thereof. In this manner, multiple (typically four) liquid crystal domains with multiple different alignment directions are defined, thereby minimizing the grayscale inversion.
Also known as another kind of VA mode is a CPA (continuous pinwheel alignment) mode. In a normal CPA mode LCD, its subpixel electrodes have a highly symmetric shape and either an opening or a projection (which is sometimes called a “rivet”) is arranged on the surface of the counter substrate in contact with the liquid crystal layer so as to be aligned with the center of a liquid crystal domain. When a voltage is applied, an oblique electric field is generated by the counter electrode and the highly symmetric subpixel electrode and induces radially tilted alignments of liquid crystal molecules. Also, with a rivet provided, the alignment control force of the slope of the rivet stabilizes the tilted alignments of the liquid crystal molecules. As the liquid crystal molecules are radially aligned within a single subpixel in this manner, the grayscale inversion can be minimized.
However, when viewed obliquely, the image displayed on a VA mode LCD will look more whitish as a whole than when viewed straight on (see Patent Document No. 1), which is called a “whitening” phenomenon. In the LCD disclosed in Patent Document No. 1, each subpixel, representing an associated one of the three primary colors of red, green and blue, has multiple regions with mutually different luminances, thereby reducing such a whitening phenomenon when the screen is viewed obliquely and improving the viewing angle characteristic. More specifically, in the LCD disclosed in Patent Document No. 1, electrodes provided for those regions of each subpixel are connected to mutually different data lines (source bus lines) by way of respectively different TFTs. The LCD of Patent Document No. 1 makes the potentials at the electrodes provided for those regions of each subpixel different from each other, thereby making those regions of each subpixel have different luminances and attempting to improve the viewing angle characteristic.
Also, even in a situation where an achromatic color is being displayed at a middle grayscale, the chromaticity may also look different depending on whether the screen is viewed straight on or obliquely (see Patent Document No. 2, for example). In the LCD disclosed in Patent Document No. 2, in a low-luminance region of each of red, green and blue subpixels, the transmittance is caused to vary in the same way as a low-grayscale level does, thereby reducing the variation in chromaticity when an achromatic color is displayed.
Nevertheless, to make those regions of each subpixel have mutually different luminances, fine electrodes should be provided for those regions of each subpixel, thus increasing the cost and sometimes resulting in a decreased yield. But a TN mode LCD can be made at a lower cost than a VA mode LCD. That is why somebody proposed that the viewing angle characteristic of a TN mode LCD could be improved even without providing multiple electrodes for each subpixel (see Patent Document No. 3, for example). Specifically, in the LCD disclosed in Patent Document No. 3, if two subpixels, which are two adjacent portions to receive the same input signal one after the other, have middle grayscale levels, then the viewing angle characteristic could be improved by setting the grayscale level of one of the two subpixels to be relatively high and that of the other subpixel to be relatively low, respectively. Specifically, supposing such two subpixels, which receive the same input signal one after the other, have middle grayscale levels A and B and the average (=L(A)+L(B)/2) of their luminances L(A) and L(B) is identified by L(X), a grayscale level X associated with that average luminance L(X) is obtained and then relatively high and low grayscale levels A′ and B′ that achieve the luminance L(X) of the grayscale level X are obtained. In this manner, the LCD disclosed in Patent Document No. 3 corrects the grayscale levels A and B represented by the input signal into grayscale levels A′ and B′, thereby attempting to improve the viewing angle characteristic without providing any such fine electrodes for each subpixel electrode.